Among pathogens of plants, fungi, responsible for fungal or cryptogamic diseases, are those which have the greatest economical impact. Each plant species is sensitive to one or several main diseases, which may strongly reduce its robustness, its growth and finally, the amounts or/and the quality of the harvest.
Various parameters influence the development of diseases such as the soil conditions and fertilization, the varietal sensitivity, the cultivation method (rotation, tillage, number of plants or seedlings per hectare, pruning system . . . ), or especially climate conditions. But acting on some of these parameters is not generally sufficient for sufficiently limiting the damages caused by the diseases. Also, as a guard against this, the practitioner who wishes to optimize and secure his/her yield, will treat his/her culture at the right time with a phytosanitary, often preventive, product. Most often, the products used are chemical products, most of which are highly efficient, but may represent health risks for personnel who handle them, and generate residues on treated productions, in the soils and waters. Further, repeated use of certain fungicidal active materials acting on the same metabolic site selects strains which are resistant to these fungicides.
In order to attempt to find a remedy to this, it is necessary to limit the number of yearly uses of chemicals of the same family, to alternate chemical families with different modes of action, and to use all other means unfavorable to the pathogen. In this context, there therefore exists a real and significant need for alternative solutions against plant diseases.
Ideally, these solutions should act in a way different from that of existing chemical fungicides, should not generate any chemical residues in the harvests and in the environment, and be safer and healthier for the operators. Such treatments should be used, either alone or alternately and/or in combination with present chemical treatments or any other treatment for preventing the occurrence or limiting the development of these pathogens and of their resistant strains, on the plants, and limit the risks for humans and the environment.
In this context, it is known how to administer antagonistic microorganisms toward pathogens to plants. For example, the microorganisms proposed hitherto as antagonistic microorganisms comprise the bacterium: Bacillus subtilis, the fungi: Trichoderma harzanium, Trichoderma viride, Coniothyrium minitans, Streptomyces griseoviridis, the yeasts: Aureobasidium pullulans, Metschnikowia fructicola, Candida oleophila . . . . The article “Biological control of post harvest diseases of fruits and vegetable” by El Ghaouth et al. (Applied Mycology and Biotechnology, Vol. 2, Agriculture and food production, Elsevier Science B.V., p. 219-238) proposes a review of what is known as regards biological control of pathogens on fruit, and proposes several possible explanations for the mode of action of antagonistic microorganisms.
The use of Saccharomyces cerevisiae yeasts as antagonistic microorganisms has also been contemplated. However, generally, the efficiency of these yeasts has been estimated as being lower than that of other microorganisms, or even in certain case nil. As an example, reference may be made to the article “Role of competition for sugars in yeasts in the biocontrol if gray mold in apples”, by A. B. Filonow (Biocontrol Science and Technology, 8:243-256, 1998), which teaches that Saccharomyces cerevisiae is inefficient for reducing infection of apples by grey mold, unlike Cryptococcus laurentii and Sporobolomyces roseus. 
Now, antagonistic microorganisms usually considered as the most efficient ones, are often isolated from the natural environment, are not well known, and may consequently express undesirable characteristics during their development or during their use. One is then faced with industrialization difficulties, or even with drawbacks toward humans or the environment.
Therefore, there exists a need for developing novel strains of antagonistic microorganisms, more reliable from an industrial point of view, safer from a health and environmental point of view, and efficient in controlling diseases caused by pathogens in plants.